1. Field of the Invention
The present invention relates to a hydraulic system for a construction machinery such as a hydraulic excavator.
2. Description of the Related Art
In a construction machinery such as a hydraulic excavator, hydraulic actuators are driven by hydraulic pressure (pressure of hydraulic fluid) obtained by driving a hydraulic pump by using a prime mover (engine, electric motor, etc.). The hydraulic actuators are small-sized, lightweight and high-power actuators and are widely used as actuators for construction machineries. In a hydraulic excavator that drives its front structure by using a hydraulic cylinder, the front structure can be moved in the gravitational direction by using the front structure's potential energy as the power for the movement, without the need of inputting hydraulic energy.
In this case, the speed of the front structure in the gravitational direction has to be controlled properly. For the control of the descending speed of the front structure, conventional construction machineries generally throttle the meter-out opening area of the control valve while dumping the hydraulic energy of the hydraulic fluid in the form of heat. In contrast, there exists a construction machinery comprising a regeneration device including a hydraulic motor and an electric motor and recovering the hydraulic energy (generally dumped in the form of heat) as electric energy by driving the electric motor with the hydraulic motor driven by the return fluid returning to the tank (see JP, A 2007-327527 (hereinafter referred to as “Patent Document 1”), for example).